Power limiting audio amplifier temperature protection circuit

Abstract

An audio amplifier of the type including circuitry to detect clipping and adjust the gain of an amplifier input element to control the clipping in closed loop operation further includes circuitry effective to sense the temperature of an amplifier component and change the amplifier bias voltage that determines the maximum non-clipping voltage so as to induce clipping in the amplifier when the temperature exceeds a predetermined temperature. The clipping control circuitry then reduces the input gain to the amplifier to control the clipping and, in so doing, reduces power dissipation to a level allowing the temperature to decrease to the predetermined temperature. The circuit provides for closed loop control of amplifier temperature in marginal temperature situations with minimal intrusion on the music reproduction process.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A protection circuit for an audio amplifier of the type having a power supply effective to supply a bias voltage determining the maximum non-clipping audio output voltage, a clipping detector effective to detect clipping in the audio output and generate a signal thereof, a circuit effective to detect the temperature of a component of the audio amplifier and generate a signal thereof and a variable gain circuit providing the input to the audio amplifier, the protection circuit comprising: first circuit elements responsive to the signal from the temperature detecting circuit to change the bias voltage supplied by the power supply to the amplifier in the direction of smaller maximum non-clipping audio output voltage when the temperature exceeds a predetermined temperature so as to induce clipping in the amplifier when the temperature of the component exceeds the predetermined temperature;   second circuit elements responsive to the signal from the clipping detector to reduce the gain of the variable gain circuit in closed loop to limit the clipping of the amplifier and thus, when clipping is initiated by the first circuit elements in response to a temperature exceeding the predetermined temperature, to reduce the power dissipation of the amplifier and the temperature thereof.   
     
     
       2. A protection circuit for an audio amplifier of the type having two amplifying circuits with bridge connected outputs, a power supply effective to supply to the amplifying circuits a full bias voltage and a half bias voltage together determining the maximum non-clipping audio output voltage, a clipping detector effective to detect clipping in the audio output and generate a signal thereof, a circuit effective to detect the temperature of a component of the audio amplifier and generate a signal thereof and a variable gain circuit providing the input to the audio amplifier, the protection circuit comprising: first circuit elements responsive to the signal from the temperature detecting circuit to change the half bias voltage supplied by the power supply to the amplifier in the direction of smaller non-clipping audio output voltage, without so changing the full bias voltage, when the temperature exceeds a predetermined temperature so as to induce clipping in the amplifier when the temperature of the component exceeds the predetermined temperature;   second circuit elements responsive to the clipping signal to reduce the gain of the variable gain circuit in closed loop to limit the clipping of the amplifier and thus, when clipping is initiated by the first circuit elements in response to a temperature exceeding the predetermined temperature, to reduce the power dissipation of the amplifier and the temperature thereof.   
     
     
       3. The protection circuit of claim 2 in which the half bias voltage is derived from the full bias voltage in a voltage divider having a pair of substantially equal resistances across the full bias voltage and the first circuit elements include a transistor biased to increase its conduction with higher temperature, above the predetermined temperature as sensed by the first circuit elements, and connected to the voltage divider to shunt current away from the one of the resistances effective to lower the half bias voltage with increasing temperature above the predetermined temperature.